We are interested in understanding how normal intestinal cells regulate their growth and how loss of that regulation results in malignant transformation. Our research focuses on molecular mechanisms by which the Src kinases contribute to the regulation. We identified RACK1 as a novel substrate and binding partner of Src, and an inhibitor of Src kinases and colonic cell growth. RACK1 regulates cell growth by suppressing Src activity at the G1 checkpoint. We hypothesize that RACK1 regulates other aspects of growth, in part via its inhibitory influence on Src. To test this hypothesis, we will: Aim 1: Further analyze mechanisms by which Src and RACK1 regulate growth of colon cells. Studies focus on their influence on cell cycle progression through Gy/W, Sam68 function during mitosis, and cell adhesion. A RACK1 mutant that is defective for suppressing Src activity will be used to distinguish Src-dependent and independent mechanisms of RACK1 function. Aim 2: Assess Src and RACK1 function in protein translation. Studies focus on mechanisms by which they regulate translational activities of key Src substrates, effectors and binding partners involved in protein synthesis. Aim 3: Analyze mechanisms by which RACK1 induces apoptosis, and Src promotes survival of colon cells. Studies focus on their influence on the intrinsic and extrinsic apoptotic pathways, and on the Akt cell survival pathway. Endogenous inhibitors of oncogenic tyrosine kinases that work at critical checkpoints in the cell cycle, at sites of cell adhesion and during protein translation and apoptosis, would exert powerful and pervasive control over cell growth. Exploitation of these multiple functions could be used to develop new and more powerful and selective strategies for treatment of human colon cancer.